Carbon dioxide is a greenhouse gas that affects the environment as well as an inexhaustible, cheap, safe and renewable carbon resource. In recent years, how to provide useful chemistry and chemical engineering products by using carbon dioxide as a carbon resource has attracted wide concern of governments and scientists while in-depth study has been carried out.
On the other hand, formamide compounds such as formamide, N, N-dimethylformamide (DMF) and N-formylmorpholine are very important chemical raw materials, solvents and pharmaceutical intermediates in which DMF is referred to as “universal solvent” which can also be used in the petrochemical industry as a gas absorbent for the separation and refining of gas; in addition, N-formyl morpholine is also an important formamide compound has a wide range of applications in industry, which can be used as a solvent in organic synthesis as well as the best extraction solvent for the separation of aromatic hydrocarbons and paraffin hydrocarbons to produce benzene with high purity (99.99%).
Commonly used methods to produce formamides in industry are formic acid method, methyl formate method, etc. The method for DMF is relatively special, which is mainly a carbon monoxide method, in which the main raw materials are synthesis ammonia, industrial methanol and carbon monoxide. At present, the raw materials for this method are of wide range of source, thus suitable for large-scale continuous production. Most of the large enterprises in United States, Japan and China use this method. However, in consideration of the large production and consumption of DMF, and the method based on non-renewable coal resources as raw material, the development of cleaner and reproducible production method is still necessary.
DMF can also be synthesized from CO2 as raw material, and formic acid obtained by hydrogen reduction of CO2 reacts with dimethylamine and is dehydrated to give DMF. But the catalyst developed later is of low efficiency, of which TON is 3400. Until 1994, Prof. NOYORI found that DMF can be synthesised in the supercritical carbon dioxide with tetra (trimethylphosphine) dichloro ruthenium complex as a catalyst at high efficiency, of which TON reached 4.2 million, but the total pressure of the supercritical reaction system is up to 210 atmospheric pressure, not only leading to high energy consumption, but also harsh requirement to the equipment and materials, so that the catalytic system is not suitable for practical applications in the industry. At the same time, the method is of poor substrate applicability due to the different solubility of amine in the supercritical carbon dioxide. Only a few formamide compounds can be synthesized in this way, such as dimethylamine, diethylamine and propylamine. In 2012, Prof. Cantat reported that polysilicate reagent can be used to reduce CO2 and reacts with several primary amine and secondary amine to prepare DMF and other formamide compounds. However, this method requires the use of equivalent amounts of silicon reagent which is of high cost, and when the substrate is a primary amine, it is prone to be over reduced, and of poor reaction selectivity.
Therefore, there is a need in the art for a new environmental friendly method for synthesizing formamide compounds which is of good substrate applicability, convenient use of catalyst, high production efficiency and suitable reaction conditions for industrial production, and low by-product pollution.